netdev - Re: [PATCH v1 net-next 02/14] net: Add a new socket option for a future transmit time.

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Date:   Thu, 28 Jun 2018 11:33:11 -0700
From:   Jesus Sanchez-Palencia <jesus.sanchez-palencia@...el.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        jan.altenberg@...utronix.de,
        Vinicius Gomes <vinicius.gomes@...el.com>,
        kurt.kanzenbach@...utronix.de, Henrik Austad <henrik@...tad.us>,
        Richard Cochran <richardcochran@...il.com>,
        Levi Pearson <levi.pearson@...man.com>,
        ilias.apalodimas@...aro.org, ivan.khoronzhuk@...aro.org,
        Miroslav Lichvar <mlichvar@...hat.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jiří Pírko <jiri@...nulli.us>,
        Richard Cochran <rcochran@...utronix.de>
Subject: Re: [PATCH v1 net-next 02/14] net: Add a new socket option for a
 future transmit time.

Hi Willem,


On 06/28/2018 07:40 AM, Willem de Bruijn wrote:
> On Thu, Jun 28, 2018 at 10:26 AM Willem de Bruijn
> <willemdebruijn.kernel@...il.com> wrote:
>>
>> On Wed, Jun 27, 2018 at 6:08 PM Jesus Sanchez-Palencia
>> <jesus.sanchez-palencia@...el.com> wrote:
>>>
>>> From: Richard Cochran <rcochran@...utronix.de>
>>>
>>> This patch introduces SO_TXTIME. User space enables this option in
>>> order to pass a desired future transmit time in a CMSG when calling
>>> sendmsg(2). The argument to this socket option is a 6-bytes long struct
>>> defined as:
>>>
>>> struct sock_txtime {
>>>         clockid_t       clockid;
>>>         u16             flags;
>>> };
>>
>> clockid_t is __kernel_clockid_t is int is a variable length field.
>> Please use fixed length fields.
> 
> Sorry, int is fine, of course, and clockid_t is used between userspace and
> kernel already.


Great. So, in addition to the other feedback in sock.c, what I'm thinking here
for the v2 is:

- move this struct to and the flags definition (as enums) to
include/uapi/linux/net_tstamp.h;

- keep clockid as a clockid_t and increase flags to u32 since this already takes
8 bytes in total anyway;

- reduce sk_clockid and sk_txtime_flags from struct sock from a u16 to a u8 each.


Thanks,
Jesus



> 
>> Also, as MAX_CLOCKS is 16, only 4 bits are needed. A single u16
>> is probably sufficient as cmsg argument. To future proof, a u32 will
>> allow for more
>> than 4 flags. But in struct sock, 16 bits should be sufficient to
>> encode both clock id
>> and flags.